The frequency of the external clock (HFXO) is specified in the datasheet as follows. (The figure below is for Leopard Gecko).
The lower limit of the frequency specified here is due to the restrictions of the crystal oscillator circuit, so this restriction does not apply to oscillators and digital clocks that do not require an oscillator circuit.
It can also be used at frequencies below 4MHz (e.g. frequencies below 1MHz).